As concerns over pollution caused by traditional fossil fuels increases and as sources of crude oil decrease, there has been increased interest in other sources of energy. One promising source of energy is the synthetic production of fuels, lubricants, and other products from natural gas (referred to at times as gas-to-liquids or GTL) preferably through the Fischer-Tropsch process. See for example U.S. Pat. Nos. 4,883,170 and 4,973,453, which are incorporated by reference herein for all purposes.
A. Introduction to the Fischer Tropsch Process
The synthetic production of hydrocarbons by the catalytic reaction of synthesis gas is well known and is generally referred to as the Fischer-Tropsch reaction. The Fischer-Tropsch process was developed in early part of the 20.sup.th century in Germany. It was practiced commercially in Germany during World War II and later has been practiced in South Africa.
The Fischer-Tropsch reaction for converting synthesis gas (primarily CO and H.sub.2) has been characterized in some instances by the following general reaction: ##STR1##
The hydrocarbon products derived from the Fischer-Tropsch reaction range from some methane to high molecular weight paraffinic waxes containing more than 50 carbon atoms.
Numerous catalysts incorporating active metals, such as iron, cobalt, ruthenium, rhenium, etc., have been used in carrying out the reaction, and both saturated and unsaturated hydrocarbons can be produced. The synthesis reaction is very exothermic and temperature sensitive whereby temperature control is required to maintain a desired hydrocarbon product selectivity.
B. Synthesis Gas Production
Synthesis gas may be made from natural gas, gasified coal, and other sources. A number of basic methods have been employed for producing the synthesis gas ("syngas"), which is substantially carbon monoxide and molecular hydrogen, utilized as feedstock in the Fischer-Tropsch reaction. The numerous methodologies and systems that have been used to prepare synthesis gas include partial oxidation, steam reforming, auto-reforming or autothermal reforming. Both fixed and fluid bed systems have been employed.
The reforming reactions are endothermic and a catalyst containing nickel is often utilized. Partial oxidation (non-catalytic or catalytic) involves sub-stoichiometric combustion of light hydrocarbons such as methane to produce the synthesis gas. The partial oxidation reaction is typically carried out commercially using high purity oxygen.
In some situations these synthesis gas production methods may be combined to form another method. A combination of partial oxidation and steam reforming, known as autothermal reforming, wherein air may be used as the oxygen-containing gas for the partial oxidation reaction has also been used for producing synthesis gas heretofore. Autothermal reforming, the combination of partial oxidation and steam reforming, allows the exothermic heat of the partial oxidation to supply the necessary heat for the endothermic steam reforming reaction. The autothermal reforming process can be carried out in a relatively inexpensive refractory lined carbon steel vessel whereby a relatively lower cost is typically involved.
The autothermal reforming process results in lower hydrogen to carbon monoxide ratio in the synthesis gas than does steam reforming alone. That is, the steam reforming reaction with methane results in a ratio of about 3:1 or higher while the partial oxidation of methane results in a ratio of less than about 2:1--depending upon the extent of the water gas shift reaction. A good ratio for the hydrocarbon synthesis reaction carried out at low or medium pressure (i.e., in the range of about atmospheric to 500 psig) over a cobalt catalyst is about 2:1. When the feed to the autothermal reforming process is a mixture of light shorter-chain hydrocarbons such as a natural gas stream, some form of additional control is desired to maintain the ratio of hydrogen to carbon monoxide in the synthesis gas at the optimum ratio (for cobalt based F-T catalysts) of about 2:1. For this reason steam and/or CO.sub.2 may be added to the synthesis gas reactor to adjust the H.sub.2 /CO ratio to the desired value with the goal of optimizing the process economics.
C. Improved Economics Desired
It has been a quest for many to improve the economics of processes utilizing the Fischer-Tropsch reaction. Improved economics will allow a wide-scale adoption of the process in numerous sites and for numerous applications. Efforts have been made toward that end, but further improvements are desirable.